swim laps not recorded properly

venu 2 's recording of my swim laps is totally off. Whenever I get 2kms done in a 50 meter pool it records, on average, 2,6 kms... yet I do set the length of the pool to 50 meters, touch the edge of the pool and push off from the wall when starting a new lap.: I don't swim fast but not slowly either (2 kms in a little bit less than 50 mns) and do not change my strokes or my pace. I am a regular swimmer whose technique is far from perfect but good enough.   any idea how to fix this issue?